In recent years the search for oil and gas has moved offshore. In order to locate potential offshore oil and gas reservoirs, it has been necessary to develop new devices and techniques for conducting marine geophysical prospecting operations. Due to the hostile environment in which they are conducted, such operations are typically quite difficult and costly to perform.
The primary method for conducting marine geophysical prospecting operations involves the use of towable marine seismic sources and seismic receiver cables. The basic principles of this prospecting method are well known to those skilled in the art. The seismic source(s) introduce seismic signals into the body of water. These signals propagate down through the water, across the water-floor interface, and into the subterranean geological formations, and are, to some extent, reflected by the interfaces between adjacent formations. The reflected signals travel upwardly through the geological formations and the body of water to a seismic receiver cable located near the surface of the body of water. The seismic receiver cable typically contains a number of hydrophones spaced along its length which record the reflected signals. Analysis of the signals recorded by the hydrophones can provide valuable information concerning the structure of the subterranean geological formations and possible oil and gas accumulation therein.
Early marine geophysical prospecting operations were generally conducted "in-line". In other words, the seismic source(s) and the seismic receiver cable were towed substantially directly behind the seismic vessel, and the resulting geophysical data was valid only for a relatively narrow band along the pathway of the vessel. Thus, the seismic vessel was required to make a number of passes along relatively closely spaced pathways in order to collect the necessary geophysical data for a given survey area. This requirement contributed directly to the cost and difficulty of conducting marine geophysical prospecting operations.
In order to reduce the number of passes of the seismic vessel necessary for any given survey area, and hence the cost of conducting the survey, the offshore geophysical industry has developed various devices and techniques for increasing the width of the "swath" of geophysical data collected during each pass of the seismic vessel. Generally such devices and techniques involve the use of multiple seismic sources and/or seismic receiver cables, each of which is towed by the seismic vessel along a discrete pathway which is parallel to but laterally spaced from the pathways of the other sources and receiver cables. Typically, the lateral spacing of the sources and receiver cables is symmetric about the pathway of the seismic vessel. See, for example, the wide seismic source disclosed in U.S. Pat. No. 4,323,989 issued Apr. 6, 1982 to Huckabee et al.
In addition to reducing the number of passes necessary for a particular survey area, the use of multiple seismic sources and/or seismic receiver cables may improve the quality of the resulting geophysical data. For example, the use of an array of seismic sources can increase the signal to noise ratio of the signal recorded by the hydrophones, thereby resulting in higher quality geophysical data. Further, the use of a plurality of seismic sources which are activated or fired simultaneously can increase the amount of energy in the seismic pulse, thereby permitting data to be gathered from very deep subterranean formations.
In order for a single vessel to tow multiple seismic sources and/or seismic receiver cables along laterally spaced parallel pathways, means must be provided for causing the objects being towed to move laterally away from the pathway of the towing vessel. One such means is disclosed in U.S. Pat. No. 4,130,078 issued Dec. 19, 1978 to Cholet. Cholet discloses a device comprising at least two parallel deflectors secured to a floating member. Each of the deflectors consists of a series of parallel paddles which are oriented obliquely to the trajectory of the device so that hydrodynamic pressure on the paddles forces the device in a lateral direction. The paddles may be either curved or flat sheets. The amount of lateral offset produced by this device is dependent on the speed that it is towed through the water, and the device cannot be remotely controlled.
Another device for laterally shifting the trajectory of a towed object is disclosed in U.S. Pat. No. 3,613,629 issued Oct. 19, 1971 to Rhyne et al. The Rhyne et al. device consists of a streamlined float with a diverter arrangement rigidly suspended below the float. Hydrodynamic pressure on the diverter causes the device to move laterally away from the pathway of the towing vessel. As with the Cholet device, the amount of lateral offset produced by the Rhyne et al. device is dependent on its speed through the water, and it cannot be remotely controlled.
Still another device for laterally shifting the trajectory of a towed object is disclosed in the above referenced patent to Huckabee et al. That device comprises an elongated float equipped with a remotely-adjustable rudder. The only lateral force generated by the Huckabee et al. device is the force resulting from hydrodynamic pressure on the rudder. Accordingly, the device is not capable of achieving large lateral offsets. Outriggers on the vessel are used to increase the maximum lateral offset produced by the device.
Submerged paravanes have been used heretofore in marine operations for a variety of purposes. For example, in commercial fishing operations submerged paravanes have been used to hold open a fishing net being towed by a vessel. Submerged paravanes have also been used in minesweeping operations to laterally shift the trajectory of the minesweeping equipment away from the pathway of the towing vessel. An example of one such submerged paravane is disclosed in U.S. Pat. No. 2,960,960 issued Nov. 22, 1960 to Fehlner. The Fehlner paravane consists of a cambered hydrofoil shaped paravane wing containing a depth control mechanism. As the paravane wing is towed through the water, the cambered hydrofoil shape generates a substantially lateral hydrodynamic force similar to the "lift" generated by an airfoil. This lateral hydrodynamic force causes the paravane wing to move laterally away from the pathway of the towing vessel. As with the surface-referenced devices described above, the amount of lateral movement is dependent on the speed of the towing vessel, and the paravane wing cannot be remotely controlled. Further, unless the paravane wing is maintained in a substantially vertical orientation, the lateral hydrodynamic force will have a vertical component which will cause the depth of the paravane wing to fluctuate.
As described above, the use of multiple seismic sources and/or multiple seismic receiver cables towed along discrete pathways parallel to but laterally spaced from the pathway of the seismic vessel may be highly beneficial in conducting marine geophysical prospecting operations, both from the standpoint of reducing the cost of conducting the survey and from the standpoint of improving the quality of the resulting geophysical data. However, the accuracy and reliability of the resulting geophysical data is dependent on precisely maintaining the lateral spacing of the various components of the system throughout the time during which the seismic vessel is traversing the survey area. Thus, the benefits resulting from the use of multiple sources and/or multiple receiver cables may be lost if the towing system is not capable of being remotely controlled and adjusted to compensate for changes in the speed of the towing vessel or variations in wind, waves, or currents. Accordingly, the need exists for a remotely-controllable device capable of maintaining the lateral offset of a towed object within certain limits over a broad range of operating conditions.